The invention relates to casings for electrochemical cells, typically cylindrical casings, and a method of forming such casings.
The casings for electrochemical cells are typically elongated and of cylindrical shape. The casings are typically in the shape of a cylinder having a closed end and open end. Conventional alkaline cells or lithium cells which are in common usage today are representative of cells employing such cylindrical casings.
Primary alkaline cell typically have a cylindrical casing which contains zinc anode active material, alkaline electrolyte, preferably aqueous potassium hydroxide, a manganese dioxide cathode active material, and an electrolyte permeable separator film, typically comprising cellulose. Such cells have a fresh voltage of about 1.5 Volt and are in widespread use. (Alkaline cells as referenced herein shall be understood to be conventional commercial alkaline cells having an anode comprising zinc, a cathode comprising manganese dioxide, and an electrolyte comprising potassium hydroxide.) Primary (non-rechargeable) lithium cells can have a cylindrical casing typically containing an electrode composite comprising an anode formed of a sheet of lithium, a cathode formed of a coating of cathode active material comprising manganese dioxide or lithiated manganese dioxide on a conductive metal substrate such as a stainless steel mesh. The lithium cells can have a sheet of electrolyte permeable separator material between the anode and cathode sheets. The separator sheet is typically placed on opposite sides of the lithium anode sheet and the cathode sheet is placed against one of the separator sheets, thereby separating the anode and the cathode sheets. The electrolyte used is typically comprises a lithium salt such as lithium trifluoromethane sulfonate (LiCF3SO3) dissolved in a nonaqueous solvent. The electrode composite is spirally wound and inserted into the cell casing, for example, as shown in U.S. Pat. No. 4,707,421. Lithium cells having a cylindrical casing can be of varying size and commonly are in the form of cylindrical cells having about ⅔ the height of a conventional AA size alkaline cell or even smaller heights. The lithium cells have a voltage of about 3.0 volts which is twice that of conventional Zn/MnO2 alkaline cells and also have higher energy density (watt-hrs per cm3 of cell volume) than that of alkaline cells. Primary lithium cells are in widespread use as a power source for many conventional photographic flash cameras, which require operation at higher voltage and at higher power output than supplied by individual alkaline cells.
The cylindrical casings for such alkaline and lithium cells have good mechanical strength and corrosion resistance and are typically of steel such as nickel plated cold rolled or nickel plated stainless steel. The cylindrical casings are formed typically from a flat sheet of metal. The metal sheet can be held in place over a block die having a cylindrical channel opening in its surface. The channel opening can run through a portion of the die""s thickness. The flat metal sheet can be drawn in a single stage or in a plurality of stages by action of a punch on the sheet until the casing of desired shape and diameter is obtained. If multiple staging is used to fabricate the casing, a series of block dies can be used each having a progressively smaller diameter channel opening. Thus, a metal sheet can be punched in the first stage to a first cup shape having a first diameter which is smaller than the diameter of the starting metal sheet. The cup product from the first stage die can be placed in a second stage die having a cylindrical channel opening of diameter less than the diameter of the first die opening. The cylindrical cup formed in the final stage is of the desired shape, diameter and length.
The casing can then be filled with active anode material, electrolyte and cathode material. An end cap assembly comprising a terminal plate with attached electrical insulator such as an insulator plug can then be placed in the open end of the casing. One of the anode or cathode is in electrical contact with the casing and the other is in electrical contact with the terminal plate. The peripheral edge of the casing at the casing open end can be crimped over the edge of the end cap assembly thereby sealing the casing with a portion the insulator between the casing and terminal plate. During crimping a portion of the casing can also be radially compressed around the end cap assembly to provide a tight seal.
It is desirable to design the cylindrical cell so that the amount of internal volume available for active material is as great as possible for a cell of given overall size. This results in increased cell capacity and service life. In order to accomplish this objective various designs of the end cap assembly, for example by flattening the end cap assembly or by using thinner insulator plugs, have been tried. Such designs have their limitations since the end cap assemblies typically include an insulator plug, and must be strong enough to withstand the crimping force needed to provide a tight seal. Another approach is to reduce the wall thickness of the casing. When conventional methods are used to form the casing, for example, by punching a flat sheet as above described, the casing wall thickness is uniform from one end of the casing to the other. If the casing is fabricated to wall thickness which is below a threshold level, the peripheral edge of the casing at the casing open end cannot be crimped effectively around the edge of the end cap assembly. For example, when the wall thickness of the casing peripheral edge thereof becomes too thin, the peripheral edge does not hold its crimped position with time but rather tends to spring back radially from its original crimped position. The xe2x80x9cspring backxe2x80x9d effect is a result of the change in physical properties occurring if the metal becomes thinned during the punching process. A casing peripheral edge which has been thinned results in a relaxation of the crimp forces around the end cap assembly and a gradual loosening of the seal between the casing and end cap assembly. This of course is undesirable since it can result in leakage of electrolyte from the cell and could also allow ambient moisture to seep into the cell. Also, if the wall thickness of the casing peripheral edge is too thin it may crack as the crimping forces are applied to it.
An aspect of the invention is directed to a casings for cylindrical electrochemical cells, for example, alkaline cells having an anode comprising zinc and a cathode comprising manganese dioxide, or lithium cells, comprising lithium metal anode and cathode comprising manganese dioxide or lithiated manganese dioxide. The casing of the invention is characterized by having a non-uniform wall thickness. The casing of the invention is not intended to be restricted to any one cell size. Thus, the casing having non-uniform wall thickness can be made with varying overall length and diameter so that it can be used as a casing for any desired cylindrical cell size, for example, AAAA, AAA, AA, C or D, ⅔ A size (same diameter as AA cell but ⅔ its length) or CR2 size (15 mmxc3x9725 mm). Thus, the casing of the invention has particular application to cells having an outside diameter between about 7 and 35 mm and a length of between about 20 mm and 60 mm. The casing comprises a cylindrical body surface, an open end and an integrally formed closed end. The closed end forms the cell bottom which functions as a cell terminal. The bottom can be flat or can have an integrally formed pip protruding from the center thereof. A peripheral edge of the casing at the open end thereof extends from the casing body. The peripheral edge desirably has a length of between 3 and 5 mm. The peripheral edge is preferably stepped so that it has an outside diameter which is greater than the outside diameter of the remainder of the casing body. After the casing is filled with active material, the casing is sealed by crimping the peripheral edge of the casing over an end cap assembly comprising a terminal plate and insulator plug.
In an aspect of the invention the casing peripheral edge has a wall thickness which is the same as the wall thickness of the casing bottom forming the closed end. Alternatively, the casing peripheral edge has a wall thickness which is even greater than the wall thickness of casing bottom. Desirably the casing body surface (excluding the casing peripheral edge) has a wall thickness which is less than both the casing bottom and peripheral edge. In a preferred embodiment the casing body surface (excluding the peripheral edge) has a wall thickness which is less than the casing bottom, and the casing peripheral edge has a wall thickness which is the same or greater than the wall thickness of the casing bottom. The casing is preferably of nickel plated steel having wall thickness between 0.003 and 0.015 inches (0.0762 and 0.381 mm). Preferably the casing bottom has a wall thickness of between about 0.006 and 0.015 inches (0.152 and 0.381 mm); the body surface (excluding peripheral edge) has a wall thickness which is between about 0.002 and 0.005 inches (0.0508 and 0.152 mm) less than the wall thickness of the casing bottom; and the peripheral edge has a wall thickness of between about 0.006 and 0.015 inches (0.152 and 0.381 mm). The body surface desirably has a wall thickness of between about 0.003 and 0.008 inches (0.0762 and 0.203 mm), preferably 0.006 inches (0.152 mm), the bottom has a wall thickness of between about 0.006 and 0.015 inches (0.152 and 0.381 mm), preferably 0.0088 inches (0.224 mm), and the peripheral edge has a wall thickness between about 0.006 and 0.015 inches (0.152 and 0.381 mm), preferably 0.009 inches (0.229 mm). The peripheral edge has a wall thickness desirably between about 0.002 and 0.006 inches (0.0508 and 0.152 mm) greater than the wall thickness of the body surface.
The casing is desirably formed by the process of the invention wherein a flat sheet of metal, preferably of nickel plated steel is first cut to a circular flat sheet in a preliminary punching step. The circular flat sheet is transferred to an intermediate punching station wherein it is drawn by punching it through a cavity within one or a series of dies. A cylindrical punch preferably of carbide steel is employed. A series of like intermediate punching steps can be used. If series of intermediate step are employed, the cup formed in one die is transferred to a next die having a cavity of reduced diameter. The cup is again punched through the die cavity in such next die with a punch of reduced diameter thereby further drawing the cup into a cup of progressively increased length and progressively reduced diameter. The tolerance between the outside surface of the punch and the inside surface of the die cavity in each of these intermediate steps is desirably between about 0.006 and 0.015 inches (0.152 and 0.381 mm) which is the range of the material thickness of the starting flat sheet. The force of the punch on the surface of the cup during the punch downstroke in each of the intermediate steps is between about 1000 and 1500 pounds force (4448 and 6672 Newtons). Such tolerance allows the cup diameter to be reduced and its length increased without altering any portion of the cup""s wall thickness. Desirably the cup""s wall thickness remains about the same in each of the like intermediate punching steps as the thickness of the starting flat metal sheet. The cup formed in the last of such intermediate punching steps is transferred to a finishing step wherein it is punched through the cavity in a finishing die. In the finishing step the cup is subjected to a one stroke action of a cylindrical punch forcing the cup through a finishing die cavity. The punching in the finishing die can further reduce the cup""s diameter and can further increase the cup""s length. As the punch, in a one stroke action, forces the cup through the finishing die cavity, the wall thickness of the cup body (exclusive of the peripheral edge) is ironed to reduce the wall thickness thereof. Thus, the wall thickness of the cup""s body (exclusive of the cup""s peripheral edge) becomes less than the wall thickness of the cup bottom, which desirably remains unaltered from the thickness of the starting sheet. Also during the same one stroke action of the punch, the wall thickness of the peripheral edge can remain unaltered or slightly increased by action of the metal being squeezed towards the cup""s open end during the punching action. The force of the punch downstroke on the surface of the cup in the finishing step is between about 1500 and 2500 pounds force (6,672 and 11, 120 Newtons). The tolerance between the punch surface and the inside surface of the die cavity in the finishing step is desirably between about 0.003 and 0.008 inches (0.0762 and 0.203 mm). Such tolerance helps to achieve the desired reduction in wall thickness of the cup""s body surface resulting in a body surface wall thickness of between about 0.003 and 0.008 inches (0.0762 and 0.203 mm). The reduction in wall thickness of the cup""s body surface increases the cell""s interior volume for a given cell size thereby allowing more active material to be inserted into the cell. The reduction in wall thickness of the cup""s body surface is accomplished without reducing the wall thickness of the cup""s peripheral edge at the cup""s open end. This makes it easier to achieve a tightly sealed cell during crimping of the peripheral edge over a terminal end cap assembly which is inserted into the cell""s open end after the casing has been filled with active material.